There is one element you need to become aware of especially at sea, and that is tidal streams. Tides don't just magically go up and down. The water flows ALONG the coast (for example on the south coast the incoming tide flows east, and the outgoing goes west). In many places you can get VERY fast tidal streams, so for example in my area (East Coast), the tidal stream will run at between 1.5-3 knot (1.8-3.6mph) along plain coast with estuary entrances going up to double that. So, if you've paddled relaxedly at at 4mph with the tide in half an hour you will have travelled 3.8 miles. Travelling back will take you NINE HOURS (actually it won't because the tide turns roughly every 6.25 hours) but it does demonstrate how fast things can go wrong.
With our boat which had a max speed of about 7 knots, we always planned to maximise the positive tidal effect otherwise the "running up the down escalator" effect is very tedious. As another example, on a big spring tide (i.e. fast tide) we travelled 17 nautical miles in 1.75 hours so we averaged 9.7kts over the ground, so give or take an average tidal stream of pretty much 3kts